The first oghams were from Ireland. In the 14th century Book of Ballymote, their invention is credited to Ogma in the Auraicept na n-Éces, or “scholars’ primer” believed to be from 7th century sources. (full text, ogham drawings start on p.300)

The introduction explains some of the secrecy of the oghams, I quote it a length as it gives a startling picture of the status of poets as a warrior class with extensive privileges. The text then goes on to give a syllabus of their subjects of study, year by year.

The poets, filid, were a guild, making their own special laws,and exercising discipline upon their own members. They claimed and used the right to quarter themselves and their retinue upon society, and they exacted a fixed sum for their poetic compositions. In general this was cheerfully paid ; the means for enforcing unwilling payment was satire. The exercise of this potent weapon was moderated by rule, certain forms of satire, such as tamall n-aire, being forbidden in the Trefhocul ; and though the poets have been abolished by law for over a century, even at this day in certain districts the phrase, dheanamh aoir air, to satirise one, is not without its terrors.

The poets were a secret society with a language peculiar and intelligible to themselves only. According to their literary tradition Feníus, at their request, devised this language for them, and its obscurity was essential.

The people often rose up against the poets and attempted to repudiate their claims. One such rising was that at Drumketta, a.d. 590. About that time they numbered 15,000. Owing to the advocacy of St Columba, himself a fili, they were suffered to continue, but under restrictions.

The filid were a strictly professional class, undergoing a rigorous training to fit them for their position. The bards, on the other hand, were unprofessional, and more or less untrained, but they practised a large number of metres in which the filid were required to become proficient.

Irish oghams aren’t the same as Pictish/Scotland oghams, but scholars have had some success interpreting some stones in Scotland. From Sir Samuel Ferguson’s, Ogham Inscriptions in Ireland, Wales, and Scotland (text):

CHAPTER VII. (Pages 133-154)
Scottish Oghams differ from those in Ireland, Wales, and England--Shetland
Oghams ; Lunnasting : St. Niniari's : Bressay--Orkney ; Burrian, Aberdeenshire ;
Newton : Logic : Aboyne--Scoonie stone in Fifeshire--Golspie in Sutherland.
210. ALL the older Oghamic monuments of Ireland, and all
those of Wales and South England, so far as they are known
to us, are of the digit and notch kind. The Oghamic monu-
ments of Scotland, on the contrary, are all of what has been
termed the scholastic variety, in which digits constitute
vowels as well as consonants, and the notch is unknown.
The stem-crossing vocalic groups are distinguished from
consonantal by being vertical to the medial line ; but this is
by no means a general rule. In some instances vowels and
consonants are sloped in reversed directions, and in some
reverse inclinations are given to both classes of letters inter
se. The consequence is a range of alternative transliterations
so wide that room can only be found for the most obvious
possible variations in the transliterated texts of this section.
The Scottish Oghams, therefore, agreeably to these views,
may be considered the more modern, and in them we may be
prepared to find more of that studied obscurity which appears
to have originated in the pedantry of later ecclesiastical
scribes. They are about equally distributed over the main-
land and the islands. In the latter we find no collateral aid
from associated epigraphs, or, save in one instance, from
definitely intelligible sculpture. On the mainland all the
examples ally themselves with peculiar Picto-Scottish forms
of sculpture, which, for such interpretation as they may
receive, require the fuller preparatory exploration.

The big surprise, for those who read the complete chapter, is that the language of the oghams in two of the stones turns out to be Old Norse. Other stones are found to have Christian symbolism, and the images accompanying the oghams on the stones are found either to be of the Wild Hunt or are compared to art in European churches with similar themes .